1. Field of the Invention
This invention pertains generally to circuit interrupters and, more particularly, to arc fault circuit interrupters. The invention also relates to methods of detecting parallel arc faults.
2. Background Information
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal, which heats and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system.
An arc fault circuit interrupter (AFCI) is a device intended to mitigate the effects of arc faults by functioning to de-energize an electrical circuit when an arc fault is detected. Non-limiting examples of AFCIs include: (1) arc fault circuit breakers; (2) branch/feeder arc fault circuit interrupters, which are intended to be installed at the origin of a branch circuit or feeder, such as a panelboard, and which may provide protection from ground faults and line-to-neutral faults; (3) outlet circuit arc fault circuit interrupters, which are intended to be installed at a branch circuit outlet, such as an outlet box, in order to provide protection of cord sets and power-supply cords connected to it (when provided with receptacle outlets) against the unwanted effects of arcing, and which may provide protection from line-to-ground faults and line-to-neutral faults; (4) cord arc fault circuit interrupters, which are intended to be connected to a receptacle outlet, in order to provide protection to an integral or separate power supply cord; (5) combination arc fault circuit interrupters, which function as either a branch/feeder or an outlet circuit AFCI; and (6) portable arc fault circuit interrupters, which are intended to be connected to a receptacle outlet and provided with one or more outlets.
During sporadic arc fault conditions, the overload capability of a conventional circuit breaker will not function since the root-mean-squared (RMS) value of the fault current is too small to activate the automatic trip circuit. The addition of electronic arc fault sensing to a circuit breaker can add one of the elements required for sputtering arc fault protection—ideally, the output of an electronic arc fault sensing circuit directly trips and, thus, opens the circuit breaker. See, for example, U.S. Pat. Nos. 6,710,688; 6,542,056; 6,522,509; 6,522,228; 5,691,869; and 5,224,006.
Arc faults can be series or parallel. Examples of a series arc are a broken wire where the ends of the broken wire are close enough to cause arcing, or a relatively poor electrical connection. Parallel arcs occur between conductors of different potential including, for example, a power conductor and a ground. Arc faults occur in series with the source and series arcs are further in series with the load. Arc faults have a relatively high impedance. Thus, a series arc results in a reduction in load current and is not detected by the normal overload and overcurrent protection of conventional protection devices. Even the parallel arc, which can draw current in excess of normal rated current in a circuit, produces currents which can be sporadic enough to yield RMS values less than that required to produce a thermal trip, or at least delay operation. Effects of the arc voltage and line impedance often prevent the parallel arc from reaching current levels sufficient to actuate the instantaneous trip function.
U.S. Pat. No. 6,522,509 discloses an arc fault detector including a current detector detecting the alternating current flowing in an electrical circuit, and a processor which generates a cumulative sum of amounts by which the alternating current in each most recent cyclic interval exceeds the current in the immediately preceding half-cycle in absolute magnitude. An arc fault indication is generated when this cumulative sum reaches a selected level. The cumulative sum is time attenuated and the arc fault indication is generated when the time attenuated cumulative sum reaches a selected level. The processor adds the calculated differential to the time attenuated cumulative sum for cyclic intervals in which the current exceeds that for the immediately preceding cyclic interval in absolute magnitude by a selected amount after a first cyclic interval in which the current exceeds a selected arming magnitude. Hence, it takes a cyclic interval with current of a magnitude above the selected magnitude to arm the system. The processor terminates adding to the attenuated cumulative sum when the sum attenuates to a predetermined minimum level. At this point, the system is disarmed and the cumulative sum is cleared.
U.S. Patent Application Publication No. 2006/0072256 discloses an “event driven” arc fault detection method that is inactive (e.g., dormant) until a current pulse is detected by a comparator. When such a current pulse occurs, an algorithm records the peak amplitude of the current pulse as determined by a peak detector circuit and an analog-to-digital converter, along with the time since the last current pulse occurred as measured by a timer. An amount equivalent to the peak amplitude of the current pulse is added to an accumulator. That accumulator amount is decayed over time. An arc fault in the power circuit is determined if the amount stored in the accumulator exceeds a predetermined value. If the peak amplitude of the current pulse is greater than a predetermined magnitude, then the algorithm is responsively activated from the inactive state. The algorithm employs a plurality of half-cycles of current flowing in the power circuit including a present half-cycle and a number of previous half-cycles. The half-cycles are defined by a current pulse having a peak amplitude of greater than the predetermined magnitude. The algorithm determines that the time to the present half-cycle from the previous half-cycle which is immediately prior to the present half-cycle is greater than a predetermined time and responsively adds a predetermined amount to an accumulator. The amount stored in the accumulator is decayed over time. An arc fault is determined in the power circuit if the amount stored in the accumulator exceeds a predetermined value.
There is room for improvement in arc fault circuit interrupters.
There is also room for improvement in methods of detecting parallel arc faults.